A better understanding of the interactions between F. tularensis and lung macrophages, the primary cells infected after inhalation, is critical to understanding the resulting disease and for the development of effective vaccination protocols. Upon pulmonary infection of mice with a lethal dose of the live vaccine strain (LVS) of F. tularensis, alveolar macrophages are the first cell type found to harbor the bacteria. The bacteria replicate within alveolar macrophage and then disseminate systematically. However, macrophage activation by IFN-gamma, or immune T lymphocytes can result in intracellular bacterial killing. Moreover, it appears that the involvement of macrophage surface receptors such as Fc receptors in the initial interaction between the bacteria and the macrophage can alter the outcome of bacterial uptake and the course of disease. These observations have led to the hypothesis that the surface receptors involved in phagocytosis of F. tularensis by alveolar macrophage as well as the activation state of the macrophage will greatly influence the ability of the infected macrophage to either kill the invading microorganism, generate and/or respond to immunoregulatory factors, or process and present F. tularensis antigens to T lymphocytes. Importantly, alterations in these parameters are likely to critically impact the clinical outcome of F. tularensis infection. The research proposed in this subproject witl test this hypothesis in 3 ways. First, the role of various macrophage cell surface receptors in the phagocytosis and intracellular killing of F. tularensis will be determined. Second, the ability of F. tularensis to stimulate and or modulate macrophage cytokine secretion, respiratory burst and nitric oxide production will be determined. Finally, the ability of macrophage invasion by F. tularensis to effect processing and presentation of F. tularensis and other antigens will be determined. The results of these experiments will provide a more complete framework in which to interpret and extend experimental results pertaining to the analysis of F. tularensis immunopathogenesis (Subproject 2), the generation of effective F. tutarensis immunity (Subproject 3) and the genetic analysis of macrophage -F. tularensis interactions.